I’m beginning to think the lfs providing the information is leading the op down a rabbit hole of failure .With chemiClean which I despise, filter needed to run. Skimmer can be turned off and should. With filter off, oxygen reduced. At minimum if turning off a filter, add an air stone for movement and oxygen exchange
The tail looks shredded suggesting aggression from a tankmate (something a clown would do)
So much information that might have been the way 30 years ago .
but so much has changed . Such as the old saying of using old sand helped skip cycle and provided good bacteria “